Gallium Nitride (GaN) is known as a material constituting a semiconductor device such as a light-emitting device. In general, a sapphire substrate as a base is used in growing a GaN-based crystal. As an example, there is known a method which includes supplying a trimethyl gallium (TMGa) gas and a nitrogen (N) gas into a reaction tube, reacting them with each other on a sapphire substrate which has been heated inside the reaction tube, and forming a GaN crystal layer on the sapphire substrate.
In addition, in order to produce a large-area GaN-based semiconductor device at low cost, a method of growing a GaN-based crystal on a silicon (Si) substrate has been proposed. However, since Si and GaN are highly reactive with each other, direct growth of the GaN-based crystal on the silicon substrate results in a melt back etching, which makes it difficult to grow a high-quality GaN crystal on the silicon substrate. To address this problem, an interlayer having high affinity with both Si and GaN is formed between the silicon substrate and the GaN crystal layer.
Further, there is known a semiconductor device in which an initial buffer region, multi-layer buffer regions and a GaN single crystal layer are sequentially stacked on a silicon substrate. The initial buffer region includes an aluminum nitride (AlN) single crystal layer. The AlN single crystal layer is formed by vapor-growing at 1100 degrees C. using a trimethyl aluminum (TMA) and ammonia (NH3) as raw material gases. In such a semiconductor device, the AlN single crystal layer as an interlayer is formed between a silicon substrate and a GaN crystal layer to prevent the melt back etching.
In the semiconductor device, the AlN single crystal layer is formed at a high film formation temperature to crystalize the aluminum nitride (AlN). However, crystallizing the aluminum nitride (AlN) requires heating the silicon substrate to a temperature close to the melting point. This may melt the silicon substrate depending upon the heating temperature. To address this problem, a process of forming an amorphous AlN film on the silicon substrate at low temperature and growing a GaN single crystal on the amorphous AlN film has been proposed. However, when the amorphous AlN film is formed at a low temperature, the amorphous AlN film is partially crystallized upon growth of the GaN crystal on the AlN film, thereby causing cracking of the AlN film. In this case, the silicon substrate reacts with the GaN through the cracks, which may result in melt back etching.